Process of etching, particularly for intaglio cylinders



Patented Nov. 19, 1935 UNITED STATES PATENT OFFICE rnocnss or ETC-RING, PARTICULARLY,

FOR mmouo CYLINDERS Irving Gurwick, New York, N. Y., assignor, by

mesne assignments,

to Shellmar Products Company, Chicago, 111., a corporation of Dela- This invention relates to etching, especially the etching of a metal by means of an acid, and more particularly the etching of intaglio printing cylinders, as through a gelatin resist.

In etching a metal by means of an acid, as in the case of etching copper intaglio cylinders through carbon tissue in the conventional manner, extreme difficulty arises by reason of the fact that variations in the rate of etching occur upon changes in temperature and humidity, thereby making it exceedingly diflicult for the etcher to accurately control the depth of etching or to gauge the same as a function of time, for the penetrability of the protective tissue and the corrosive action of the acid are functions of temperature and humidity.

The general object of the present invention is to overcome this difliculty. More particular objeets of the invention are to make possible an exceedingly slow, rate of etch when desired; to make possible an accurately controlled depth of etch obtained at a relatively rapid rate by first etching to any desired depth at a rapid rate and then readily slowing up the rate of etch and continuing the etching process at so gradual a rate that it may be closely Watched and terminated exactly at the desired depth; and to make the depth of etch dependent upon the etching time and substantially independent of changes in temperature or/and humidity.

It is customary to screen the light sensitized tissue in addition to exposing the same to the image to be reproduced. In etching solid areas to a substantial depth considerable difliculty arises by reason of the fact that the acid besides eating downwardly into the metal, tends to eat sidewardly into the cross lines of the screen, which weakens and may break down the screen. Accordingly, still another object of the present invention is to overcome this difiiculty and to make it possible to etch solids to any desired depth while using as sharp or fine a screen as may be desired without, however, destroying or breaking down the screen.

The carbon tissue is ordinarily sensitized in a light sensitive solution such as potassium bichromate, and the absorption of the solution by the tissue tends to vary with weather conditions; and still another object of my invention is to eliminate this variable factorand to make it possible to obtain uniform results when sensitizing the carbon tissue, without resorting to a costly expedient such as complete temperature and humidity control of the plant.

To the accomplishment of the foregoing and such other objects as will hereinafter appear, my invention consists in the etching process and the steps thereof, and their relation one to the other as hereinafter are more particularly described in the specification and sought to be defined in the claims.

In the etching of intaglio cylinders it is the practice to protect the cylinder by or to etch through a previously sensitized and photographicallyv exposed carbon tissue. This resistor tissue may consist of a heavy grade of soft paper impregnated or coated on one side with gelatin and glycerine, and preparatory to use it is dipped in or coated with a light sensitive solution, usually potassium bichromate until the gelatin absorbs the solution and therefore itself becomes light sensitive. The sensitized tissue is later placed in a vacuum frame together with a screen of desired flnene'ss and exposed to light, after which it is placed in the vacuum frame with a previously photographically made positive transparency of required design and again exposed to light. The sensitized and exposed tissue is then moistened and placed face down on an intaglio roller or cylinder, and the gelatin resist is developed and the paper backing removed therefrom in conventional manner, the soft portions of the gelatin being washed away and the hardened portions of the gelatin remaining, so that the resist consists of relatively thin and relatively thick areas. The cylinder is then coated with an etching fluid, the parts of the cylinder lying beneath the thin areas of the resist being quickly etched, whereas the parts of the cylinder or plate lying beneath the thick areas of the resist are protectively coated thereby.

I shall, for convenience, refer to the. etching fluid as an etching acid as it is ordinarily called, but it will be understood that a conventional fluid such as ferric chloride may be used and is meant even though it is not strictly an acid. The acid may be applied to the copper plate or cylinder by immersing the latter in an acid bath or by painting the same with a soft wad of cotton or the like dipped in the acid. The etcher watches 5 the changes in color of the copper as an indication or measure of the depth of etch. Acids of different strengths are ordinarily prepared and employed in an endeavor to control the etch produced for the various tones of reproduction, and it is important that the etching be carried through as one continuous process from beginning to end, for the resist must be kept wet with the etching fluid until the process is completed.

It is, of course, highly desirable that the rate of etch be standardized or made uniform so that the depth of etch may be gauged as a function of time. However, this has not heretofore been possible because the rate of etch depends upon the rate of penetration of the resist by the acid, and this in turn varies greatly with temperature and humidity conditions for an acid of given strength. It also varies greatly with the strength of the acid. The problem is complicated by the fact that a strong acid penetrates the resist more slowly than a weak acid. The etcher first applies a strong acid for penetration of the shadows or thin portions of the resist, and when these have been approximately sufficiently etched he changes to a weaker acid in order to accelerate penetration of the thicker portions of the resist, thereby accelerating the etching process. It has not been possible to stop and thus completely control the etching process. I have found that the desired result may be obtained by the use of a very cold acid, for the penetration and rate of etch of such an acid is slow and may be reduced to any desired low value, which in turn makes it possible for the etcher to watch the progress of the etching and to accurately control the same.

I have also discovered that the rate of etch obtained when using a very cold acid is substantially constant for a given temperature, and that at the relatively low temperatures in question humidity becomes a far less important factor, so that the depth of etch may be gauged as a function of time. For this the acid should, of course, be kept cold, while in use.

It is further found that the exceedingly slow rate of etch obtained by the use of a very cold acid'may be used as a brake or control factor for accurately controlling the ultimate depth of etch obtained, even if'the etch is started rapidly by the use of a warm or normal acid. More particularly, it is feasible and desirable for the etcher to coat the cylinder with any desired acid for rapid penetration of the resist, and to watch the progress obtained. When the copper is attacked the etcher may change from the first acid to a chilled or refrigerated acid, which immediately arrests the rapid etching action but does not stop the etching process. Instead, the etching is continued at a very slow or gradual rate and it may be closely watched until the exact desired depth is reached, at which time the process may be stopped altogether by washing the etching cylinder in the usual manner. The protective gelatin coating may finally be removed, all in accordance with the usual practice.

Now, in accordance with the present invention, all these advantages and conveniences are made readily available, for I have discovered and applied an exceedingly inexpensive and convenient way to obtain the desired greatly chilled acid. My invention centers most importantly about this method, which is to simply place in a body or bath of the acid some solid carbon dioxide or dye ice as it is colloquially termed. The carbon dioxide volatilizes in the acid and rapidly abstracts heat therefrom, bringing the acid down to any desired low temperature which may be measured by a suitable thermometer immersed in the acid. No special containers are needed for the acid, for ordinary treated wood tanks'are satisfactory. Problems such as would arise with refrigerating systems, such as attack on metal cooling coils by the etching acid, are not even met with in my preferred process. Expensive machinery is dispensed with, and no extra floor space or plant equipment is necessary. The volatile product being simply inert carbon dioxide, bubbles through the acid and leaves the same without reacting chemically with the etching acid or in any way deleteriously affecting the same or changing its concentration. Such a chilled body 5 of acid may be used by the etcher either for the entire etching process or better for the finishing stages of the etching process, 'as has already been explained. For a given low temperature ranging from, say, zero to 40 F., the rate of etch is substantially dependent upon the time of etching, and this fact is exceedingly helpful to the etcher. At the same time changes in humidity become of little importance particularly when the acid is used at a rather low temperature, so that by foll5. lowing the present process it is possible to obtain many of the advantages of using a temperature and humidity controlled plant without undertaking the expense of the costly equipment needed for such a plant. Furthermore the present invention makes it possible to use temperatures far lower than could be maintained in even a plant having temperature and humidity control, because of the obvious discomfort to the workers.

In etching a solid area on an intaglio cylinder the area is not really entirely etched away but instead a screen is left formed by the heretofore mentionedexposure of the carbon tissue to a screen of desired fineness. This screen -is protected by the hardened lines of the resist but nevertheless is attacked to some extent by the acid used for etching because the acid while working downwardly, also tends (though to a less extent) to work sidewardly and thereby to weaken the screen. I have discovered that when the acid is brought down to a low temperature, as by the use of solid carbon dioxide as above described, the etcher can secure almost any desired depth of etch without breaking down the screen and regardless of how sharp or fine the desired screen may be. The etching rate is, of course, reduced, but the rate at which the acid tends to undercut the screen is not simply proportionately reduced but is reduced to a still greater amount.

It seems that the cold acid is less active and operates so gradually that the protective coating succeeds in more effectively protecting the copper surface against the cold acid than is the case with a more corrosive warm acid. I am not certain of the theoretical explanation for this penomenon but submit the same as an empirical fact which I have discovered.

I prefer to rotate the cylinder in a bath of etching acid, the entire bath being kept cold by the solid carbon dioxide. A temperature of 30 to 40 F. is convenient to use, but still lower temperatures are feasible and sometimes desirable. I have, for example, etched with acid of 10 F. for several hours until a depth of fifteen thousandths of an inch was reached without breaking down a 150 line screen. Both the depth and screen fineness then obtained were abnormally and unnecessarily great, but they illustrate the possibilities of my method.

The invention is thus of great value for etching 5 solids. The thickness of the resist often varies even when exposed to a positive transparency having uniform solid areas. An etching depth of 0.0035 inch maybe sufiicient for solid areas, but to obtain this depth uniformly would require controlling the etchings at the different portions of the resist to compensate for inequalities therein. A far more convenient method is to simply let the etch go to a greater depth, say, 0.005

inch, in which case slight variations will be immaterial and a solidly inked reproduction will be obtained, the inequalities being covered up; but etching to this depth tends to break down the screen, or 'to so greatly thin the edges thereof that these edges may be flattened after a short period of active printing with the cylinder. By using a very cold etching acid, as above described, this problem is solved.

The invention is also of value when etching reproductions of pictures or the like in which there are many gradations of tone. For example, when the etcher finally comes to'etch the high lights, he can control the etching process by applying the, cold acid and using the cold acid for very slow etching until the exact desired slight depth of etch is reached.

Not only is the present invention of value in the etching process for the step of etching a metal by means of an acid, but it also is of value in the earlier step of sensitizing the carbon tissue by means of a light sensitive solution. The carbon tissue is impregnated in. a solution such as potassium bichromate, with the object of absorbing the solution so as to make the gelatinous substance of the tissue itself light sensitive. This absorption should be uniform in amount but unfortunately varies with temperature and humidity conditions. Furthermore the absorption at a warm temperature is spotty and irregular, spoiling the results finally obtained. In accordance with the present invention the potassium bichromate or other light sensitive solution may first be chilled by placing a quantity of solid carbon dioxide in the solution until a predetermined cool temperature is reached, say, 40 or at most 50 F. The solid carbon dioxide may then be removed and the carbon tissue immersed and soaked in. the solution for a predetermined period of time, say,

one-half an hour. In the meantime, if the temperature of the solution tends to change greatly, the dry ice may again be immersed therein. However, slight changes in temperature such as ordinarily will take place during the soaking period do not seriously affect the rate of absorption of the solution and may ordinarily be neglected.

Here again the use of dry ice is a simple and inexpensive expedient which may be adopted in any existing establishment and is even better than temperature and humidity control in that it makes possible the use of a temperature consi'd erably lower than can be experienced with physical comfort by the workers.

It is believed that the process of my invention and the method of applying the same to the etching of metals in general and to the etching of intaglio cylinders in particular, as well as the many advantages thereof, will be apparent from the foregoing detailed description. The etching acid or bath is chilled quickly, in fact, almost instantly, so that no delay is involved in providing the desired cold acid. The acid may be chilled while lying directly in the bath in which the intaglio cylinder is to be immersed. The acid may be lowered to very low temperatures. These low temperatures may be obtained or maintained tage of using separate chilled and unchilled process. 5

No excessively large quantity of acid must be preliminarily chilled and made available. No extra equipment or elaborate equipment of any kind is required. Noproblem of acid attack upon metal parts is raised. The greatly chilled acid 10 may be used for slow etching, or as a brake to slow up rapid etching, or as a method of obtaining exceedingly deep etching without breaking down the screen lines. Air conditioning ordinarily intended to keep the resist free from moisl5 ture may be dispensed with because when cold acid is used it in itself serves to strengthen the resist and to prevent that rapid penetration which might otherwise be permitted due to moisture absorption by the resist. The very cold a id used serves to chill the resist and also the intaglio cylinder.

It will further be understood that while I have described the process of my invention in preferred form, many changes and modifications may be made in the steps thereof without departing from the spirit'of the invention, defined in the following claims.

I claim:

1. In the etching of intaglio cylinders, the method which includes placing solid carbon dioxide in a body of etching acid in order to reduce the temperature of the acid to between 30 F. and 40 F., and thereafter coating the cylinder with the chilled etching acid.

2. In the etching of intaglio cylinders through carbon tissue, the. method which includes applying the already sensitized and exposed carbon tissue to the cylinder in the conventional manner, placing solid carbon dioxide in a body of etching acid in order to greatly reduce the temperature of the acid, and thereafter coating the cylinder with the chilled etching acid, the acid being chilled to a predetermined low temperature resulting in a predetermined low rate of etching not materially affected by variations in atmospheric temperature and humidity conditions.

3. In the etching of intaglio cylinders by means of an acid applied through resist or tissue, the method of accurately controlling the final depth of etch, which includes first etching the cylinder with acid of moderate or warm temperature in order to penetrate the resist, thereupon changing over to a chilled acid in order to arrest the rapid etching and to continue the etching at a very gradual rate, stopping the application of the chilled acid when the etch has reached the exact desired depth, and finally removing the protective material from the cylinder.

4. In etching intaglio cylinders, the method of accurately controlling the depth of etch, which includes preliminarily preparing two bodies of I etching acid, placing a quantity of solid carbon dioxide in one of the bodies of acid in order to greatly chill the same, first applying the unchilled acid to the cylinder and etching with said acid,

' thereupon applying the chilled acid and continuing the etching very slowly by means of the chilled acid until the exact desired depth is reached. I

5. In etching intaglio cylinders through resist or tissue, the method of accurately controlling the depth of etch, which includes preliminarily preparing two bodies of etching acid, placing a quantity of solid carbon dioxide in one of the bodies 76 in order to greatly chill the same, protecting the cylinder with the tissue, applying the unchilled acid to the tissue until the tissue is penetrated and the etch begun, thereafter applying the 5 chilled acid and continuing the etching very slowly by means 'of the chilled acid until the exact desired depth is reached.

6. In the etching of-solid areas on an intaglio cylinder by means of an etching fluid applied through a preliminarily screened carbon tissue or resist, the method of obtaining a desired deep etch without breaking down or undercutting the screen, which includes etching with an etching fluid having a temperature substantially lower than 40 F.

7. In the etching of solid areas on an intaglio cylinder by means of an etching fluid applied through a preliminarily screened carbon tissue or resist, the method of obtaining a desired deep etch without breaking down or undercutting the screen, which includes first applying to the resist an etching fluid adapted to readily penetrate the thin areas of the resist, and thereafter continuing the etching process with an etching fluid having a temperature substantially lower than 8. In the etching of solid areas on an intaglio cylinder by means of an etching fluid applied through a preliminarily screened carbon tissue or resist, the method of obtaining a desired deep etch without breaking down or undercutting the screen, which includes placing solid carbon dioxide in an etching bath to reduce its temperature to a value below 40 F., first applying to the resist an etching fluid of normal temperature adapted to penetrate the thin areas of the resist, and thereafter continuing the etching process with the chilled etching fluid having a tem: perature lower than F.

9. In the etching of solid areas on an intaglio cylinder by means of acid appl ed through pre- 5 liminarily screened carbon tissue the method of obtaining the desired depth of etch without breaking down the screen, which includes pre liminarily placing solid carbon dioxide in the etching acid to greatly chill the same, and there- 10 after applying the chilled acid to the cylinder.

10. In the etching of intaglio printing cylinders through carbon tissue, the method which includes applying the already sensitized and exposed carbon tissue to the cylinder in the conventional 15 manner, cooling a body of etching solution substantiaily below normal temperature by placing therein a quantity of a material which is normally a gas but which has been reduced in temperature sufliciently to effect a change in physical 0 state, which material when placed in the etching solution will not substantially change the chemical nature of the solution or afiect the concentration thereof, and treating said cylinder with said greatly cooled etching solution. 25

11. The method of etching metals through resist or tissue, which comprises applying the tissue to the metal in the conventional manner, cooling a body of etching solution substantially below normal temperature by placing therein a solidi- 30 fled gas which when passing from the solid to the gaseous state will not substantially change the chemical nature of the etching solution or affect the concentration thereof, and treating said metal with the solution. 35

IRVING GURWICK. 

